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Sound Transit looked deeper into the light-rail tunnels and bridges that neighborhoods from Ballard to West Seattle requested since the 2016 election that approved transit expansion, and the numbers are breathtaking.

Some proposed routes would add more than $300 million. A few would add years of delay — the most extreme a tunnel through two ridges to reach West Seattle Junction. That could cost $1.2 billion more than the proposed elevated track.

It’s been almost two years since voters approved property-tax, car-tab and sales-tax increases to pay for the Sound Transit 3 plan of eight regional rail extensions and two bus rapid-transit lines.

Proposed ST3 routes

Sound Transit will display and discuss its preliminary findings about new light-rail routes at these at neighborhood forums:

People may also review the information and comment online at www.wsblink.participate.

Source: Sound Transit

Traffic Lab is a Seattle Times project that digs into the region’s thorny transportation issues, spotlights promising approaches to easing gridlock, and helps readers find the best ways to get around. It is funded with the help of community sponsors Alaska Airlines, CenturyLink, Kemper Development Co., NHL Seattle, PEMCO Mutual Insurance Company and Seattle Children’s hospital. Seattle Times editors and reporters operate independently of our funders and maintain editorial control over Traffic Lab content.

Five layers of advisory and political review groups are overlapping this year, in what’s called an accelerated effort to write a plan. Risks are mounting of cost overruns and delays — beyond the current land and labor inflation that’s hitting some transit projects.

As of this week, eight route options remain in Ballard, four downtown, seven in Sodo and the Chinatown International District, and five in West Seattle. The list needs to be winnowed down in October, or planning will fall behind schedule, staffers say.

Consultants performed 1,200 comparisons, he said, to reflect pros and cons of different options. The transit board allocated some $285 million and five years to choose the final routes and finish environmental studies, before final engineering and construction begin.

Some highlights, from north to south:

• Fixed-bridge options are getting a prolonged look in Ballard, where staff is investigating a 136-foot-high span comparable in height to the Aurora Bridge. The ST3 measure showed a cheaper 70-foot-high drawbridge that might need to open two to four times a day and delay trains.

• Tunnels from Interbay to Ballard could cost $300 million to $500 million more than the elevated trackway in the original ST3 plan. A Ballard station could be 120 feet deep, or three stories lower than the existing UW Station.

• Some options provide a Magnolia station along 20th Avenue West near West Dravus Street, which would require a bridge across BNSF Railway’s Ballmer freight yard.

• A Seattle Center train stop might go under Mercer Street, with entrances on both sides of the road.

• A popular South Lake Union (SLU) proposal moves the second SLU stop to Fifth Avenue North and Harrison Street, rather than cramming two stations east of Aurora Avenue, three blocks apart. The Fifth Avenue site offers quick access to the RapidRide E Line bus, the Space Needle and the Bill & Melinda Gates Foundation. But the path would require tunneling under KeyArena.

• Deep-bore tunnel options at Union Station in the Chinatown International District, situated amid tideflats and fill soil, would require boarding platforms 200 feet deep. Such depths would force a station planned near Madison Street and Fifth Avenue some 250 feet below street level.

• The original and cheapest Chinatown International District scenario entails a shallow cut-and-cover tunnel beneath Fifth Avenue South. That’s frequently criticized for inflicting havoc on people who have already suffered through their share of construction to build the First Hill streetcar line as well as other projects.

• A surface trackway in Sodo could save $300 million — and still provide road overpasses at Lander and Holgate streets to separate trains from general traffic and trucks.

• A Pigeon Ridge tunnel to West Seattle would reduce elevated tracks among Delridge and Avalon Way homes, and put Delridge Station farther south near more transit riders. But reaching the ridge would require crossing the Argo freight-rail yard and passing high-voltage electrical lines, before drilling into a steep slope, said Stephen Mak, corridor development manager. That’s could cost an extra $1.2 billion.

• Among less-extreme routes, a short tunnel being considered from the proposed Delridge Station to the corner of Fauntleroy Way Southwest and Southwest Alaska Street would still add $700 million.

If all these questions can be resolved, Seattle riders might someday gain 14 new stations, to serve an estimated 210,000 daily trips, in what’s currently the fastest growing U.S. transit market.